The present application relates generally to variable speed drives. The application relates more specifically to an input filter for a variable speed drive to filter line to ground voltages and reduce high frequency distortion in the converter stage.
A variable speed drive (VSD) for heating, ventilation, air-conditioning and refrigeration (HVAC&R) applications typically includes a rectifier or converter, a DC link, and an inverter. The rectifier or converter converts the fixed line frequency, fixed line voltage AC power from an AC power source into DC power. The DC link filters the DC power from the converter and typically contains a large amount of electrical capacitance. Finally, the inverter is connected in parallel with the DC link and converts the DC power from the DC link into a variable frequency, variable voltage AC power.
If a high frequency distortion of the line to ground voltage (Vlg) is present at the input of the VSD, the voltage across the input devices (Vce) can become distorted. The distortion of Vce may cause high voltage transients across the input devices of the VSD in addition to the normal input voltage to the VSD. If the high voltage transients combined with the input voltage exceed the maximum permissible rated voltage for an input device of the VSD, the device may fail. A failure can occur when the input devices are not switching or operating, but are still connected to the input lines.
The traditional solution to the problem of high frequency distortion of Vlg is to provide a galvanically isolated disconnect, e.g., a contactor, between the VSD and AC power source. The galvanically isolated disconnect isolates the input devices from the high frequency distortion of Vlg, when the input devices are not switching. The galvanically isolated disconnect increases the cost, size, complexity and weight of the VSD. Another solution to mitigate high frequency distortion of Vlg may be to use input devices rated at a higher voltage. Devices with higher voltage ratings, however, increase the cost of the VSD and may reduce its efficiency.
What is needed is a system and/or method that satisfy one or more of these needs or provides other advantageous features. The present application is directed to both VSDs that incorporate an active converter type AC to DC converter topology and VSDs utilizing conventional AC to DC rectifier converters.
Other features and advantages will be made apparent from the present specification. The teachings disclosed extend to those embodiments that fall within the scope of the claims, regardless of whether they accomplish one or more of the aforementioned needs.